1. Field of the Invention
The present invention relates to methods used for managing the memory used for storing messages in a digital telephone answering device, and more particularly to methods by which empty or unused gaps in memory may be removed to leave two types of contiguous memory: used and unused.
2. Description of the Prior Art
Telephone answering devices have become commonplace in recent years, and have taken such advanced forms as voice-mail networks. Most former telephone answering devices (TADs) were relegated to the use of cassette tapes for outgoing and incoming message recording and playback. Recently, advances in digital technology as applied to TADs have made possible the use of random access memory (RAM) to store and retrieve outgoing and incoming messages. Such digital TADs, or DTADs, require supporting circuitry that converts analog signals to digital signals and vice-versa. Beyond this supporting circuitry, DTADs are similar to regular, cassette-based TADs.
Arising with the use of RAM in DTADs, comes more flexibility of use. This flexibility includes the deletion of specific messages in random order while allowing the saving of other specific messages, also in random order. As the messages are stored digitally, the deletion of messages from memory and the transfer of messages between memory locations becomes greatly facilitated, especially compared to the previous cassette tape storage means.
When messages are deleted from RAM in random order, unused gaps of memory appear when a deletion is made between two saved messages. This memory gap cannot be used for further message storage as any outgoing or incoming message is of arbitrary length, depending on how long the person speaks. If the time available for the outgoing or incoming message is always the same, this may pose no problem. However, current technology allows TADs to record an incoming message for as long as the caller speaks. Further, as all messages are stored in sequential order in memory, the use of such a gap requires either overwriting part or all of a saved message, or the use of complex bookkeeping of pertinent memory locations so that outgoing or incoming messages saved in the space vacated by deleted messages can be saved piecemeal to preserve the saved messages. The latter alternative of piecemeal storage requires a great deal of "hopscotching" around message memory and ultimately leads to confusion within the controlling circuits.
With the advent of digital technology in the use of TADs, has also come increased expense. Digital circuitry requires greater design considerations as well as the use of more expensive components. For these reasons, the amount of available digital memory used in DTADs is not as great as the available cassette tape memory of older TADs. The approximately thirty minutes of memory present on cassette tape costs approximately ten cents per minute while five to ten minutes of digital memory present in DTADs costs about eight dollars per minute. Due to memory and cost limitations, the efficient use of this limited and expensive digital memory becomes an important consideration.
In Hashimoto's U.S. Pat. No. 4,821,311, issued Apr. 11, 1989, a DTAD is disclosed having a very limited incoming message recording time of about one and one-half minutes for about 1.5 megabits of memory. However, Hashimoto does allow the deletion of messages stored in digital memory in any order, while allowing other messages to be saved. In some embodiments, Hashimoto discloses memory apportionment by chip location, one chip per message. Recovery of deleted message memory is then implemented by moving the contents of a saved message into the memory chip of the deleted message. This results in some realization of efficient memory use, but the same could be accomplished by storing the order in which messages come in and the chips in which the messages are stored. Further, as there is a single message in each memory chip, a message longer than the time allotted is clipped at its end and a message shorter than the time allotted leaves some memory within the chip unused.
In other Hashimoto embodiments, memory apportionment is accomplished on a contiguous basis using CPU registers as pointers to memory locations. For the second case, Hashimoto does not disclose any explicit method for retrieving deleted memory space for efficient future use. Instead, mere analogy is used to the method for chip-wise message storage.
In U.S. Pat. No. 4,856,051 issued to Ohtawara et al. on Aug. 8, 1989, the only memory saved is memory accidentally used to record a silent incoming message, such as when the caller hangs up and the DTAD continues to record the telephone line signal. When such incoming message silence is detected, Ohtawara erases the message to recover the memory used. This only works for the latest message taken by the DTAD, and does not foresee memory recovery for deleted messages lying between two saved messages.
In Millet, U.S. Pat. No. 4,794,638, issued on Dec. 27, 1988, means for memory management are disclosed that use one chip per message and an array of pointers that keep track of the chronological order of the messages. Such means do not provide for the case of contiguous memory and may lose the use of some memory space when messages are stored that do not completely fill the allotted chip. Further, messages longer than the time allowed by an allotted chip do not have the overflow portion recorded.
Accordingly, there is a need for the efficient use of RAM when used in digital telephone answering machines for the recording of outgoing or incoming messages. Further, a need for the efficient use of DTAD RAM is present that maximizes the recovery of deleted message memory space so that the vacated memory space can be used to record future messages.